Synaptic plasticity is considered essential for learning and storage of new memories. Whether all synapses on a given neuron have the same ability to express long-term plasticity is not well understood. Synaptic microanatomy could affect the function of local signaling cascades and thus differentially regulate the potential for plasticity at individual synapses. Here, we investigate how the presence of endoplasmic reticulum (ER) in dendritic spines of CA1 pyramidal neurons affects postsynaptic signaling. We show that the ER is targeted selectively to large spines containing strong synapses. In ER-containing spines, we frequently observed synaptically triggered calcium release events of very large amplitudes. Low-frequency stimulation of these spines induced a permanent depression of synaptic potency that was independent of NMDA receptor activation and specific to the stimulated synapses. In contrast, no functional changes were induced in the majority of spines lacking ER. Both calcium release events and long-term depression depended on the activation of metabotropic glutamate receptors and inositol trisphosphate receptors. In summary, spine microanatomy is a reliable indicator for the presence of specific signaling cascades that govern plasticity on a micrometer scale.long-term depression ͉ metabotropic glutamate receptor ͉ metaplasticity ͉ spine apparatus ͉ dendritic spines A ctivity-dependent changes in synaptic strength are thought to be essential for learning and the formation of new memories (1). The intracellular signaling cascades underlying different forms of synaptic plasticity have been studied extensively at the CA3 to CA1 projection in the hippocampus. Although long-term potentiation at these synapses is strictly NMDA receptor-dependent, at least two mechanistically distinct forms of long-term depression (LTD) have been described, triggered by the activation of NMDA receptors (NMDARs) and metabotropic glutamate receptors (mGluRs), respectively (2). Although the potential for NMDAR-dependent plasticity can be regulated by the subunit composition of the receptor itself, much less is known about the regulation of mGluR-dependent plasticity (3). Aberrant mGluR signaling and dysregulated synaptic plasticity have been implicated in severe mental disorders, such as fragile X mental retardation (4). The induction of mGluRdependent LTD is known to involve activation of postsynaptic group I mGluRs and inositol trisphosphate (IP 3 )-mediated calcium release from the endoplasmic reticulum (ER) (reviewed in ref. 5). Interestingly, only a small subset of dendritic spines on CA1 pyramidal cells contains ER (6). The heterogeneous distribution of this organelle very well could affect the plasticity of individual synapses (7,8).In all previous studies of synaptic depression, plasticity was induced at large numbers of synapses simultaneously. However, this strategy does not allow the investigation of functional differences between individual synaptic connections. Differences in synaptic microanatomy, such as the presen...

Dendritic spines have been proposed to function as electrical compartments for the active processing of local synaptic signals. However, estimates of the resistance between the spine head and the parent dendrite suggest that compartmentalization is not tight enough to electrically decouple the synapse. Here we show in acute hippocampal slices that spine compartmentalization is initially very weak, but increases dramatically upon postsynaptic depolarization. Using NMDA receptors as voltage sensors, we provide evidence that spine necks not only regulate diffusional coupling between spines and dendrites, but also control local depolarization of the spine head. In spines with high-resistance necks, presynaptic activity alone was sufficient to trigger calcium influx through NMDA receptors and R-type calcium channels. We conclude that calcium influx into spines, a key trigger for synaptic plasticity, is dynamically regulated by spine neck plasticity through a process of electrical compartmentalization.

GABA B receptors are the G-protein-coupled receptors for GABA, the main inhibitory neurotransmitter in the brain. GABA B receptors are abundant on dendritic spines, where they dampen postsynaptic excitability and inhibit Ca 2+ influx through NMDA receptors when activated by spillover of GABA from neighboring GABAergic terminals. Here, we show that an excitatory signaling cascade enables spines to counteract this GABA B -mediated inhibition. We found that NMDA application to cultured hippocampal neurons promotes dynamindependent endocytosis of GABA B receptors. NMDA-dependent internalization of GABA B receptors requires activation of Ca 2+ /Calmodulindependent protein kinase II (CaMKII), which associates with GABA B receptors in vivo and phosphorylates serine 867 (S867) in the intracellular C terminus of the GABA B1 subunit. Blockade of either CaMKII or phosphorylation of S867 renders GABA B receptors refractory to NMDA-mediated internalization. Time-lapse two-photon imaging of organotypic hippocampal slices reveals that activation of NMDA receptors removes GABA B receptors within minutes from the surface of dendritic spines and shafts. NMDA-dependent S867 phosphorylation and internalization is predominantly detectable with the GABA B1b subunit isoform, which is the isoform that clusters with inhibitory effector K + channels in the spines. Consistent with this, NMDA receptor activation in neurons impairs the ability of GABA B receptors to activate K + channels. Thus, our data support that NMDA receptor activity endocytoses postsynaptic GABA B receptors through CaMKIImediated phosphorylation of S867. This provides a means to spare NMDA receptors at individual glutamatergic synapses from reciprocal inhibition through GABA B receptors.γ-aminobutyric acid | spines | trafficking | synaptic plasticity | GABAB

Long-term potentiation (LTP), a form of synaptic plasticity, is a primary experimental model for understanding learning and memory formation. Here, we use light-activated channelrhodopsin-2 (ChR2) as a tool to study the molecular events that occur in dendritic spines of CA1 pyramidal cells during LTP induction. Two-photon uncaging of MNI-glutamate allowed us to selectively activate excitatory synapses on optically identified spines while ChR2 provided independent control of postsynaptic depolarization by blue light. Pairing of these optical stimuli induced lasting increase of spine volume and triggered translocation of ␣CaMKII to the stimulated spines. No changes in ␣CaMKII concentration or cytoplasmic volume were observed in neighboring spines on the same dendrite, providing evidence that ␣CaMKII accumulation at postsynaptic sites is a synapse-specific memory trace of coincident activity.channelrhodopsin-2 ͉ dendritic spines ͉ synaptic plasticity ͉ two-photon uncaging ͉ MNI-glutamate A ctivity-dependent changes in synaptic strength are generally considered to be the cellular basis of learning and memory (1). Long-term potentiation (LTP), the most extensively studied form of such synaptic plasticity, can be triggered within seconds by coincident activity in presynaptic and postsynaptic cells. The possible structural modifications that occur at synapses where LTP has been induced are poorly known because of the difficulty of simultaneously measuring functional and morphological parameters at individual synapses. Furthermore, it is controversial whether neighboring synapses can be modified independently (2-4). In a recent report, it has been shown that spatially clustered synapses can cooperate in the induction of plasticity and that cytoplasmic factors are responsible for this functional cross-talk (5). The identity of these diffusible factors, however, has not been clarified.A key player in the LTP signaling cascade is ␣CaMKII, which is thought to function as a molecular switch: Following activation by Ca 2ϩ -calmodulin, it can stay active for prolonged periods of time via autophosphorylation (6, 7). Reports that brief application of glutamate or NMDA to cultured hippocampal neurons induces CaMKII accumulation in spines (8-10) have created much interest because ␣CaMKII activation is both necessary and sufficient to induce synaptic plasticity (6, 11). It has been suggested that postsynaptic accumulation of ␣CaMKII could be responsible for the synapse-specificity of LTP, because it localizes the putative activated kinase close to its substrates, e.g., AMPA receptors (7, 12) and protects it from dephosphorylation (13). However, a crucial prediction of this hypothesis, namely that ␣CaMKII accumulates specifically and exclusively at synapses that undergo LTP, has never been tested experimentally.To address whether ␣CaMKII accumulates specifically in spines experiencing coincident activity, we developed an alloptical pairing protocol to induce synaptic plasticity at identified spines, combining two-photon uncaging of ...

Spike timing-dependent long-term potentiation (t-LTP) is the embodiment of Donald Hebb's postulated rule for associative memory formation. Pre-and postsynaptic action potentials need to be precisely correlated in time to induce this form of synaptic plasticity. NMDA receptors have been proposed to detect correlated activity and to trigger synaptic plasticity. However, the slow kinetic of NMDA receptor currents is at odds with the millisecond precision of coincidence detection. Here we show that AMPA receptors are responsible for the extremely narrow time window for t-LTP induction. Furthermore, we visualized synergistic interactions between AMPA and NMDA receptors and back-propagating action potentials on the level of individual spines. Supralinear calcium signals were observed for spike timings that induced t-LTP and were most pronounced in spines well isolated from the dendrite. We conclude that AMPA receptors gate the induction of associative synaptic plasticity by regulating the temporal precision of coincidence detection.orrelated activity in connected neurons can trigger longlasting changes in synaptic strength, in which sign and magnitude of synaptic modifications depend on the relative timing of pre-and postsynaptic action potentials (1-3). Presynaptic activity followed by postsynaptic action potentials generally leads to an increase in synaptic strength (timing-dependent long-term potentiation, t-LTP), whereas activity in the reverse order induces long-term depression. Remarkably, the existence of t-LTP was predicted >60 y ago by the Canadian psychologist Donald Hebb as a mechanism for associative learning (4). Although t-LTP is considered a crucial mechanism for activity-dependent modifications of brain circuits, the biophysics of coincidence detection are not fully understood. The required coincidence detector needs to measure the relative timing of postsynaptic action potentials (APs) with respect to the brief glutamate transient in the synaptic cleft with millisecond precision and to convert this temporal measurement into a synapse-specific biochemical signal. Postsynaptic NMDA receptors (NMDARs), due to their sensitivity to both glutamate and membrane depolarization, have been proposed to act as detectors of temporal coincidence. Ca 2+ influx through NMDARs activates a series of biochemical processes that eventually lead to strengthening of the synaptic connection (5-9). However, there is a striking mismatch between the slow kinetics of NMDARs and the very brief time window in which t-LTP can be induced, suggesting that an additional mechanism is necessary to sharpen the timing sensitivity (10-12).Here we investigate the role of AMPA receptors (AMPARs) during coincidence detection at Schaffer collateral synapses. Modulation of AMPAR currents during coincident activity strongly affected the induction of synaptic plasticity by pairing of preand postsynaptic spikes. Furthermore, we visualized NMDARdependent calcium signals in individual spines of CA1 pyramidal cells. During pairing stimulation, AMPAR ...

Genes of the Polycomb group (PcG) are part of a cellular memory system that maintains appropriate inactive states of Hox gene expression in Drosophila. Here, we investigate the role of PcG genes in postembryonic development of the Drosophila CNS. We use mosaic-based MARCM techniques to analyze the role of these genes in the persistent larval neuroblasts and progeny of the central brain and thoracic ganglia. We find that proliferation in postembryonic neuroblast clones is dramatically reduced in the absence of Polycomb, Sex combs extra, Sex combs on midleg, Enhancer of zeste or Suppressor of zeste 12. The proliferation defects in these PcG mutants are due to the loss of neuroblasts by apoptosis in the mutant clones. Mutation of PcG genes in postembryonic lineages results in the ectopic expression of posterior Hox genes, and experimentally induced misexpression of posterior Hox genes, which in the wild type causes neuroblast death, mimics the PcG loss-of-function phenotype. Significantly, full restoration of wild-type-like properties in the PcG mutant lineages is achieved by blocking apoptosis in the neuroblast clones. These findings indicate that loss of PcG genes leads to aberrant derepression of posterior Hox gene expression in postembryonic neuroblasts, which causes neuroblast death and termination of proliferation in the mutant clones. Our findings demonstrate that PcG genes are essential for normal neuroblast survival in the postembryonic CNS of Drosophila. Moreover, together with data on mammalian PcG genes, they imply that repression of aberrant reactivation of Hox genes may be a general and evolutionarily conserved role for PcG genes in CNS development.

No abstract